title: Fast repair of O6-ethylguanine, but not O6-methylguanine, in transcribed genes prevents mutation of H-ras in rat mammary tumorigenesis induced by ethylnitrosourea in place of methylnitrosourea.
abstract: Differential repair of structurally distinct mutagenic lesions in critical genes may influence the cellular risk of malignant conversion. We have investigated rat mammary tumorigenesis induced by N-ethyl-N-nitrosourea (EtNU) versus N-methyl-N-nitrosourea (MeNU) with respect to tumor incidence, ras gene mutation, and gene-specific repair. Both carcinogens induced mammary adenocarcinomas at high yield. In mammary epithelia (very low expression of O6-alkylguanine-DNA alkyltransferase, MGMT), O6-methylguanine (O6-MeGua) was eliminated from transcribed (H-ras and beta-actin) and inactive genes (IgE heavy chain) at the same slow rate as determined for bulk genomic DNA. The persistence of O6-MeGua in DNA correlated with a high frequency of G:C --> A:T transition mutations at codon 12 of the H-ras gene in MeNU-induced tumors. Repair of O6-ethylguanine (O6-EtGua), too, was slow in the IgE heavy chain gene as in bulk DNA. Contrasting with O6-MeGua, however, O6-EtGua was removed approximately 20 times faster from the active H-ras and beta-actin genes via MGMT-independent mechanism(s). Accordingly, no H-ras codon 12 mutations were found in EtNU-induced tumors, and 5- to 8-fold surplus alkyltransferase activity of the mammary epithelia-via a bacterial ada transgene-did not significantly counteract tumorigenesis in EtNU-exposed contrary to MeNU-treated animals. Neither MeNU- nor EtNU-induced tumors exhibited mutations at codons 13 and 61 of H-ras or codons 12, 13, and 61 of K-ras. Fast repair of O6-EtGua, but not O6-MeGua, in transcribed genes thus prevents mutational activation of H-ras when rat mammary carcinogenesis is initiated by EtNU in place of MeNU.
authors.affiliation: Institute of Cell Biology (Cancer Research), University of Essen Medical School and West German Cancer Center Essen, Hufeland-Strasse 55, D-45122 Essen, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Engelbergs
authors.forename: J
authors.initials: J
authors.lastname: Thomale
authors.forename: A
authors.initials: A
authors.lastname: Galhoff
authors.forename: M F
authors.initials: MF
authors.lastname: Rajewsky
mesh_terms: D000478:Alkylation; D000818:Animals; D030801:Animals, Genetically Modified; D018736:DNA Adducts / Q000378:metabolism*; D004260:DNA Repair; D004273:DNA, Neoplasm / Q000235:genetics; D005038:Ethylnitrosourea; D005260:Female; D011905:Genes, ras; D006147:Guanine / Q000031:analogs & derivatives* / Q000378:metabolism; D008321:Mammary Glands, Animal / Q000378:metabolism; D008325:Mammary Neoplasms, Experimental / Q000139:chemically induced / Q000235:genetics*; D008770:Methylnitrosourea; D019853:O(6)-Methylguanine-DNA Methyltransferase / Q000378:metabolism*; D017354:Point Mutation; D051381:Rats; D014158:Transcription, Genetic
chemical_list: D018736:DNA Adducts; D004273:DNA, Neoplasm; C007673:6-ethylguanine; D006147:Guanine; D008770:Methylnitrosourea; D019853:O(6)-Methylguanine-DNA Methyltransferase; D005038:Ethylnitrosourea